1. Technical Field of the Invention
This invention broadly relates to machining a workpiece. The invention more particularly relates to cutting threads on a workpiece. The invention still more particularly relates to a method for cutting internal threads on the surface of a hole drilled in metal. The invention specifically relates to a tapping composition and to a method of using the composition to make internal threads on the surface of a hole drilled in metal.
2. Description of the Prior Art and Problems Solved
It is known that xe2x80x9ctappingxe2x80x9d is a method employed in the machining art for cutting internal threads in a workpiece, such as a metal body. Tapping is, thus, broadly comprised of the steps of drilling a hole of desired diameter in a metal body; selecting a solid linear article, i.e., a tap, having a circular cross section and desired grooves formed on the exterior surface of the tap, wherein the outside diameter of the grooves on the tap is greater than the diameter of the hole drilled in the metal body; rotating the selected tap; and inserting the rotating tap into the previously drilled hole whereby the grooves on the tap contact and cut into the sides of the hole in the metal body to thereby produce the desired internal threads.
Contact between the grooves of the rotating tap and the sides of the hole is accompanied by the generation of frictional heat. This heat, if not properly handled, can result in the production of threads having undesirable quality, possible damage to the metal body, damage to the tap and shortened tap life. The art has sought methods for preventing or dissipating the mentioned frictional heat to avoid, or at least to reduce, the problems caused by such heat.
The art has, accordingly, developed various compositions, referred to as lubricants, to aid in the performance of the tapping process, wherein a lubricant is placed on the tap grooves prior to the insertion of the tap into the hole to be tapped. The purpose of the lubricant is to reduce frictional resistance between the rotating tap and the wall of the hole to help reduce the generation of frictional heat and to dissipate heat generated during the process.
It is believed that existing tapping lubricants tend to develop undesirably low viscosity during the tapping process and tend to drain away from the tap and, thus, cease to function as desired to reduce friction and dissipate heat. As a result, thread quality is low and tap life is unacceptably shortened. A method of measuring tap life is the number of holes which can be threaded with the same tap.
It is obvious that a quantity of power is required to cause a tap to rotate in a hole in order to cut threads. In this regard, it has been observed that the higher the power required to produce threads the lower the tap life. Skilled operators take as a signal that a new tap is required when the power required to tap a hole approaches a certain value. Thus, in order to save power, to produce threads of acceptable quality and to avoid tap failure, skilled operators know to change taps when the mentioned power threshold is approached.
It is thus an object of this invention to provide a tapping composition which can be employed in a tapping process to increase tap life, to produce threads of acceptable quality, and to conserve power.
This invention comprises a tapping composition, a method of making the composition and a method of using the composition to make internal threads on the surface of a hole drilled in a workpiece, such as a flat metal plate or a metal block. The tapping composition of this invention is comprised of a uniform blend of a glyceride, a liquid hydrocarbon oil and a polyvinyl chloride resin. A process of making the tapping composition of this invention is disclosed and, in one aspect, the composition is the product of that process.
It has been observed that use of the tapping composition of this invention to cut threads has required the expenditure of power in an amount less than that required to cut threads with identical taps under identical conditions when employing lubricants known in the prior art. This observation has been confirmed by an observed increase in tap life. Without being bound by a particular theory of operation, it is believed that the composition of this invention, due to the generation of heat, forms a film on the tap and that the film does not drain away from the essential points of contact between the grooves of the tap and the wall of the hole.